Silicon nitride substrate and silicon nitride circuit board using the same

ABSTRACT

A silicon nitride substrate including silicon nitride crystal grains and a grain boundary phase and having a thermal conductivity of 50 W/m·K or more, wherein, in a sectional structure of the silicon nitride substrate, a ratio (T 2/ T 1 ) of a total length T 2  of the grain boundary phase in a thickness direction with respect to a thickness T 1  of the silicon nitride substrate is 0.01 to 0.30, and a variation from a dielectric strength mean value when measured by a four-terminal method in which electrodes are brought into contact with a front and a rear surfaces of the substrate is 20% or less. The dielectric strength mean value of the silicon nitride substrate can be 15 kV/mm or more. According to above structure, there can be obtained a silicon nitride substrate and a silicon nitride circuit board using the substrate in which variation in the dielectric strength is decreased.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

Embodiments described hereinbelow generally relate to a silicon nitridesubstrate and a silicon nitride circuit board using the same.

2. Description Of Related Art

In recent years, the application of silicon nitride (Si₃N₄) substratesto semiconductor circuit boards is being attempted. Alumina (Al₂O₃)substrates and aluminum nitride (AlN) substrates are used assemiconductor circuit boards. Although an alumina substrate has athermal conductivity of around 30 W/m·K, the use thereof enables areduction in costs. Further, in the case of an aluminum nitridesubstrate, it is possible to achieve high thermal conductivity in whichthe thermal conductivity becomes 160 W/m·K or more. On the other hand,substrates having a thermal conductivity of 50 W/m·K or more are beingdeveloped as silicon nitride substrates.

Although a silicon nitride substrate has a low thermal conductivitycompared to an aluminum nitride substrate, the silicon nitride substratehas a superior three-point bending strength of 500 MPa or higher. Thethree-point bending strength of an aluminum nitride substrate isnormally between around 300 and 400 MPa, and there is a tendency for thestrength to decrease as the thermal conductivity increases. It ispossible to make use of this advantage of high strength to reduce thethickness of a silicon nitride substrate. Since it is possible to lowerthe thermal resistance by thinning the substrate, heat dissipation (heatradiating property) is improved.

A silicon nitride substrate that makes uses of such characteristics hasbeen widely used as a circuit board by providing a circuit portion suchas a metallic plate thereon. Further, a method is also available inwhich a silicon nitride substrate is used as a circuit board for apressure-contact structure, as described in International PublicationNo. WO 2011/010597 (Patent Document 1).

Thermal conductivity, strength and also insulating properties can bementioned as examples of characteristics required of a silicon nitridesubstrate for the various usages described above.

A silicon nitride substrate with favorable insulating properties isproposed in Japanese Patent Laid-Open No. 2002-201075 (Patent Document2). In Patent Document 2, a silicon nitride substrate is disclosed inwhich a current leakage value is 1000 nA or less when an alternatingvoltage of 1.5 kV and 100 Hz is applied between a front surface and arear surface of the silicon nitride substrate under conditions of atemperature of 25° C. and a humidity of 70%. A lower value for thecurrent leakage value indicates higher insulating properties between thefront and rear surfaces.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: International Publication No. WO 2011/010597    pamphlet-   Patent Document 2: Japanese Patent Laid-Open No. 2002-201075

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, there have been cases in which the insulating properties areinsufficient even if the current leakage value is equal to or less thana fixed value as described in Patent Document 2. As the result ofthoroughly investigating the cause of the problem, it was found that anabundance ratio of silicon nitride crystal grains to a grain boundaryphase in the thickness direction of the substrate is significantlyrelated to the aforementioned situation. A silicon nitride substrate ismade from a silicon nitride sintered body that comprises silicon nitridecrystal grains and a grain boundary phase. In comparison to the grainboundary phase, the silicon nitride crystal grains have superiorinsulating properties. Therefore, a portion in which the insulatingproperties differ according to the abundance ratio of the grain boundaryphase is formed within the silicon nitride substrate. Consequently, evenif the current leakage value is equal to or less than a fixed value, aphenomenon occurs such that the insulating properties becomeinsufficient.

Means for Solving the Problems

A silicon nitride substrate according to an embodiment is a siliconnitride substrate comprising silicon nitride crystal grains and a grainboundary phase and having a thermal conductivity of 50 W/m·K or more, inwhich, in a sectional structure of the silicon nitride substrate, aratio (T2/T1) between a total length T2 of the grain boundary phase anda thickness T1 of the silicon nitride substrate is between 0.01 and0.30, and a variation from a dielectric strength mean value as measuredby a four-terminal method in which electrodes are brought into contactwith the front and rear surfaces of the substrate is 20% or less.

Advantage of the Invention

In the silicon nitride substrate according to an embodiment, the ratio(T2/T1) of the total length T2 of the grain boundary phase with respectto the thickness T1 of the silicon nitride substrate is defined within apredetermined range, and hence variations in the insulating propertiesin the thickness direction are small. Therefore, in the case of usingthe silicon nitride substrate in a circuit board or the like, a highlyreliable circuit board with excellent insulating properties can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating an example of thesectional structure of a silicon nitride substrate according to anembodiment.

FIG. 2 is a cross-sectional view for describing a ratio (T2/T1) of atotal length T2 of a grain boundary phase with respect to a substratethickness T1 in the silicon nitride substrate according to theembodiment.

FIG. 3 is a cross-sectional view illustrating an example of a method formeasuring a dielectric strength according to a four-terminal method withrespect to a silicon nitride substrate.

FIG. 4 is a plan view illustrating an example of locations for measuringthe dielectric strength.

FIG. 5 is a side view illustrating one example of a method for measuringa volume resistivity value of a silicon nitride substrate.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A silicon nitride substrate according to the present embodiment is asilicon nitride substrate comprising silicon nitride crystal grains anda grain boundary phase and having a thermal conductivity of 50 W/m·K ormore, in which, in a sectional structure of the silicon nitridesubstrate, a ratio (T2/T1) between a total length T2 of the grainboundary phase and a thickness T1 of the silicon nitride substrate isfrom 0.01 to 0.30, and a variation from a dielectric strength mean valueas measured by a four-terminal method in which electrodes are broughtinto contact with the front and rear surfaces of the substrate is 20% orless.

The silicon nitride substrate is made from a silicon nitride sinteredbody that comprises silicon nitride crystal grains and a grain boundaryphase and has a thermal conductivity of 50 W/m·K or more. Preferably thethermal conductivity is 50 W/m·K or more, and more preferably is 90W/m·K or more. If the thermal conductivity is a low value of less than50 W/m·K, heat dissipation decreases.

FIG. 1 illustrates an example of the sectional structure of the siliconnitride substrate according to the embodiment. In FIG. 1, referencenumeral 1 denotes a silicon nitride substrate, reference numeral 2denotes silicon nitride crystal grains, reference numeral 3 denotes agrain boundary phase, and reference character T1 denotes a thickness ofthe silicon nitride substrate.

Further, FIG. 2 is a cross-sectional view for explaining a ratio (T2/T1)of the total length T2 of the grain boundary phase with respect to thesubstrate thickness T1 in the silicon nitride substrate according to theembodiment. In FIG. 2, reference numeral 2 denotes the silicon nitridecrystal grains, reference numeral 3 denotes the grain boundary phase,and reference characters T2-1 to T2-4 denote respective lengths of thegrain boundary phase in the thickness direction.

The silicon nitride substrate is made from a silicon nitride sinteredbody that comprises silicon nitride crystal grains and a grain boundaryphase. Preferably, among the silicon nitride crystal grains, a numberproportion of β-Si₃N₄ crystal grains is 95% or more and 100% or less.When the number proportion of the β-Si₃N₄ crystal grains is equal to orgreater than 95%, a structure is obtained in which silicon nitridecrystal grains are randomly present, and the strength is thus improved.

The grain boundary phase is mainly constituted by a sintering aid. Oneor more kinds selected from rare earth elements, magnesium, titanium andhafnium are preferable as a sintering aid. Preferably, the respectivesintering aids are contained in a total of 2 to 14 mass % in terms ofoxide content. If the amount of a sintering aid is less than 2 mass % interms of oxide content, there is a risk that a portion will arise atwhich the abundance ratio of the grain boundary phase is small. Further,if the amount of a sintering aid is an excessive amount that exceeds 14mass % in terms of oxide content, there is a risk that the abundanceratio of the grain boundary phase will be too large. Therefore, it ispreferable that sintering aids are contained within a range of 4.0 to12.0 mass % in terms of oxide content.

A feature of the sectional structure of the silicon nitride substrate ofthe embodiment is that a ratio (T2/T1) of the total length T2 of thegrain boundary phase with respect to the thickness T1 of the siliconnitride substrate is from 0.01 to 0.30. The thickness T1 of the siliconnitride substrate is the thickness of the substrate as shown in FIG. 1.The thickness T1 of the substrate is measured by calipers.

The method for measuring the total length T2 of the grain boundary phasewill now be described with reference to FIG. 2. FIG. 2 is across-sectional view for explaining the ratio (T2/T1) of the totallength T2 of the grain boundary phase with respect to the substratethickness T1 in the silicon nitride substrate. In FIG. 2, referencenumeral 2 denotes the silicon nitride crystal grains, and referencenumeral 3 denotes the grain boundary phase. First, an enlargedphotograph is taken of an arbitrary sectional structure in the thicknessdirection of the silicon nitride substrate. If the sectional structurein the thickness direction cannot be observed in one field of view, anoperation to photograph the sectional structure can be divided into aplurality of photographing operations.

Preferably, the enlarged photograph is a scanning-type electronmicroscope (SEM) photograph. In the case of an SEM photograph, there isthe advantage that it is easy to distinguish silicon nitride crystalgrains and the grain boundary phase because a contrast difference arisestherebetween.

With regard to the magnification, distinction of silicon nitride crystalgrains and a grain boundary phase is performed with ease if themagnification is 2,000 times or more. To determine the total length T2of the grain boundary phase, a straight line is drawn in the substratethickness direction on the enlarged photograph of the sectionalstructure, and the length of a grain boundary phase that is present onthe straight line is determined.

In the case illustrated in FIG. 2, the total of T2-1, T2-2, T2-3 andT2-4 is the value of T2 (T2=(T2-1)+(T2-2)+(T2-3)+(T2-4)). In the case ofphotographing an enlarged photograph by performing a plurality ofphotographing operations, the operation is repeated until the substratethickness T1 is photographed. Note that, when taking an enlargedphotograph, photographing is performed after subjecting an arbitrarycross section to mirror polishing to achieve a surface roughness Ra of0.05 μm or less, and then performing an etching process. Either ofchemical etching and plasma etching is effective as the etching process.Further, pores present in the substrate are not counted as part of thelength of the grain boundary phase.

A feature of the sectional structure of the silicon nitride substrate ofthe embodiment is characterized in that a ratio (T2/T1) of the totallength T2 of the grain boundary phase with respect to the thickness T1of the silicon nitride substrate is ranging from 0.01 to 0.30. If theratio (T2/T1) is less than 0.01, the insulating properties will decreasebecause regions in which the grain boundary phase is small will bepartially formed. On the other hand, if the ratio (T2/T1) exceeds 0.30,it will cause variations to arise in the insulating properties becauseregions in which the grain boundary phase is large will be partiallyformed. In order to ensure both the insulating properties and thereduction of such variations, preferably the aforementioned ratio(T2/T1) is within a range of 0.10 to 0.25.

By defining the aforementioned ratio (T2/T1) within a range of 0.10 to0.30 in this manner, a variation from a dielectric strength mean valueas measured by a four-terminal method in which electrodes are contactedagainst the front and rear surfaces of the substrate can be made 20% orless, and furthermore 15% or less.

FIG. 3 illustrates an example of a method for measuring the dielectricstrength using the four-terminal method. In FIG. 3, reference numeral 1denotes a silicon nitride substrate, reference numeral 4 denotes a frontsurface-side measurement terminal, reference numeral 5 denotes a rearsurface-side measurement terminal, and reference numeral 6 denotes ameasurement instrument. A tip of the front surface-side measurementterminals 4 and the rear surface-side measurement terminals 5 is aspherical shape. By making the tip of the measurement terminals aspherical shape, it is possible to make the contact pressure onto thesilicon nitride substrate 1 a constant pressure, and measurement errorscan thus be eliminated.

The front surface-side measurement terminals 4 and the rear surface-sidemeasurement terminals 5 are disposed facing each other in a manner inwhich the silicon nitride substrate 1 is sandwiched therebetween.Regardless of which positions the front surface-side measurementterminals 4 and rear surface-side measurement terminals 5 are arrangedat on the silicon nitride substrate 1, a variation from the dielectricstrength mean value of the silicon nitride substrate 1 of the embodimentis 20% or less.

The aforementioned dielectric strength mean value is a mean value thatis determined by measuring at least five locations on the siliconnitride substrate 1 by the aforementioned measurement method. Oneexample of locations for measuring the dielectric strength isillustrated in FIG. 4. For example, in the case of measuring onesubstrate at five locations, the five locations S1, S2, S3, S4 and S5that are shown in FIG. 4 are adopted as the measurement locations. Thatis, a point Si that is a point of intersection (center) between diagonallines of the substrate 1, and the four points S2 to S5 that aremidpoints between S1 and the respective corner portions are adopted asthe five locations.

The mean value of the dielectric strength at these five measurementpoints is adopted as the dielectric strength mean value of the siliconnitride substrate 1. That is, when the dielectric strength at S1 istaken as “ES1”, the dielectric strength at S2 is taken as “ES2”, thedielectric strength at S3 is taken as “ES3”, the dielectric strength atS4 is taken as “ES4”, and the dielectric strength at S5 is taken as“ES5”, a dielectric strength mean value ESA is determined by thefollowing equation. Further, at least five measurement points are used,and the number of measurement points may be six or more.ESA=(ES1+ES2+ES3+ES4+ES5)/5

A variation (%) in the dielectric strength is determined as an absolutevalue of a proportion (%) of a deviation from the mean value by (|meanvalue ESA-ESn|/ESA)×100(%); where n=integer (measurement point number).Note that, with respect to measurement conditions other than thosedescribed above, measurement is performed in accordance withJIS-C-2141.Further, measurement of the dielectric strength is performedin Fluorinert. Fluorinert is a perfluorocarbon (PFC)-based insulatingsolvent.

Variations in the dielectric strength of the silicon nitride substrateof the embodiment are small amounts of 20% or less.

The silicon nitride substrate is a silicon nitride sintered body madefrom silicon nitride crystal grains and a grain boundary phase. Whenused as a substrate, the silicon nitride substrate of the embodiment isused as a thin substrate in which the substrate thickness is 1.0 mm orless, and further, is 0.4 mm or less. This is because, the thermalresistance of the substrate is reduced and the heat dissipation isincreased by thinning the substrate.

In a thin substrate in which the aforementioned substrate thickness T1is 1.0 mm or less, if partial variations in the dielectric strength arelarge, the electric field is liable to concentrate at a portion at whichthe dielectric strength is low. Consequently, there is a concern that aportion at which the dielectric strength is low is liable to cause adielectric breakdown. In the silicon nitride substrate of theembodiment, since variations in the dielectric strength are reduced, theelectric field can be effectively prevented from concentrating at aportion at which the dielectric strength is low. Therefore, it is alsopossible to thin the substrate thickness T1 up to a thickness of 0.1 mm.In other words, the silicon nitride substrate of the embodiment iseffective as a thin substrate in which the thickness T1 is ranging from0.1 to 1.0 mm, and further, is from 0.1 to 0.4 mm.

Preferably, the dielectric strength mean value ESA is 15 kV/mm or more.If the mean value is less than 15 kV/mm, the insulating properties as asubstrate will be insufficient. Preferably, the dielectric strength meanvalue ESA is 15 kV/mm or more, and more preferably is 20 kV/mm. If theabove described ratio (T2/T1) is made 0.15 or less, it is easy for thedielectric strength mean value to be 20 kV/mm or more.

Preferably, a volume resistivity value when a voltage of 1,000 V isapplied at room temperature (25° C.) is 60×10¹² Ωm or more. Further,preferably a ratio (ρv2/ ρv1) between a volume resistivity value ρv1when a voltage of 1,000 V is applied at room temperature (25° C.) and avolume resistivity value ρv2 when a voltage of 1,000 V is applied at250° C. is 0.20 or more.

FIG. 5 illustrates a method for measuring a volume resistivity value. InFIG. 5, reference numeral 1 denotes the silicon nitride substrate,reference numeral 7 denotes a front surface-side carbon electrode,reference numeral 8 denotes a rear surface-side carbon electrode, andreference numeral 9 denotes a measurement instrument. Note that, whenmeasuring the volume resistivity value, the silicon nitride substrate 1is pressed and immobilized by the front surface-side carbon electrode 7and the rear surface-side carbon electrode 8. Further, the appliedvoltage is set as a direct current of 1000 V, and a volume resistance Rvafter the voltage is applied for 60 seconds is measured. The volumeresistivity value is determined by the equation: volume resistivityvalue ρv=Rv·πd2/4t. Where, π represents the circular constant (=3.14), drepresents the diameter of the front surface-side carbon electrode, andt represents the thickness of the silicon nitride substrate. A valueobtained by measurement of the volume resistivity value in this mannerat room temperature (25° C.) is taken as ρv1, and a value obtained bymeasurement in a 250° C. atmosphere is taken as ρv2. Further, withrespect to measurement conditions other than those described above,measurement is performed in accordance with JIS-K-6911.

Preferably, the volume resistivity value when a voltage of 1,000 V isapplied at room temperature (25° C.) is 60×10¹² Ωm or more. It ispossible to mount various semiconductor elements on a silicon nitridecircuit board in which a metallic circuit plate is provided on thesilicon nitride substrate.

There are some semiconductor elements having a high operating voltage of500 to 800 V. Preferably, ρv1 is 60×10¹² Ωm or more, and furtherpreferably is 90×10¹² Ωm or more. By increasing the volume resistivityvalue upon decreasing variations in the dielectric strength as describedabove, excellent reliability can be obtained such that a dielectricbreakdown does not occur even if a semiconductor element with a highoperating voltage is mounted.

Further, by making the ratio (ρv2/ρv1) a high ratio of 0.20 or more, orfurthermore, 0.40 or more, excellent insulating properties can bemaintained even under a usage environment of a high temperature of 200to 300° C. In recent years, semiconductor elements, such as SiCelements, are being developed for which the operating temperature isfrom 150 to 250° C. By using the silicon nitride substrate according tothe embodiment as an insulating substrate that mounts such semiconductorelements, excellent reliability can also be obtained as a semiconductordevice.

Further, when a cross section in the thickness direction of the siliconnitride substrate is observed with an enlarged photograph, preferably amaximum length of the grain boundary phase is 50 μm or less. Further,preferably the average long grain diameter of the silicon nitridecrystal grains is 1.5 to 10 μm. In order to make the dielectric strengthmean value a high value and make a variation therefrom a value that is20% or less, it is effective to set the abundance ratio (T2/T1) betweenthe silicon nitride crystal grains and the grain boundary phase in thethickness direction to a value within a predetermined range.

In addition, it is effective to control the size of the grain boundaryphase for setting the volume resistivity value ρv1 to a predeterminedvalue or more and for setting the ratio (ρv2/ρv1) to a predeterminedvalue or more. When a cross section in the thickness direction of thesilicon nitride substrate is observed with an enlarged photograph,preferably the maximum length of a grain boundary phase is 50 μm orless, more preferably is 20 μm or less, and further preferably is 10 μmor less. The term “maximum length of the grain boundary phase in thethickness direction” refers to each of the aforementioned T2-1, T2-2,T2-3 and T2-4 being 50 μm or less.

Further, in order to set the maximum length of the grain boundary phaseto be 50 μm or less, it is preferable that the average long graindiameter of the silicon nitride crystal grains is formed to be 1.5 to 10μm. The long diameter of the silicon nitride crystal grains isdetermined by measuring the maximum diameter of respective siliconnitride crystal grains that are photographed within a unit area of 100μm×100 μm in an enlarged photograph of an arbitrary sectional structureof the silicon nitride substrate, and adopting a mean value thereof asthe long diameter of the silicon nitride crystal grains.

Measurement of the maximum diameter is performed by taking the longestdiagonal line in silicon nitride crystal grains that appear in theenlarged photograph as the long diameter. This operation is performed inunit areas of 100 μm×100 μm at three different locations, and a meanvalue is taken as the average long grain diameter of the silicon nitridecrystal grains.

If the aforementioned average long grain diameter of the silicon nitridecrystal grains is a small diameter of less than 1.5 μm, grain boundariesbetween the silicon nitride crystal grains will increase, so that thereis a risk that portions at which the ratio (T2/T1) exceeds 0.30 will beformed. If the average long grain diameter of the silicon nitridecrystal grains is a large diameter that exceeds 10 μm, although thenumber of grain boundaries between the silicon nitride crystal grainswill decrease, the lengths of the grain boundaries between the siliconnitride crystal grains will increase, so that there will be a risk thata portion at which the maximum length of the grain boundary phase cannotbe made 50 μm or less will be formed. Therefore, the average long graindiameter of the silicon nitride crystal grains is preferably set towithin a range of 1.5 to 10 μm, and more preferably set to within arange of 2 to 7 μm. Note that, an enlarged photograph that is magnifiedby 2,000 times or more is used. Further, in a case where it is difficultto determine crystal grains and a grain boundary, an enlarged photographthat is magnified by 5,000 times is used.

It is preferable that the porosity of the silicon nitride substrate is3% or less. Further, the maximum diameter of the pores is 20 μm or less.In the silicon nitride substrate according to the embodiment, since theratio (T2/T1) between the silicon nitride crystal grains and the grainboundary phase in the substrate thickness direction is controlled, evenif the porosity is up to 3%, variations in the dielectric strength canbe kept to 20% or less, and furthermore 15% or less.

Note that it is preferable to keep the amount of pores as low aspossible, and the porosity is preferably set to 1% or less, and morepreferably is 0.5% or less. Further, the maximum diameter of the poresis preferably set to 20 μm or less, more preferably 10 μm or less, andfurther preferably is 3 μm or less (including 0). The maximum diameterof the pores is determined based on an enlarged photograph at anarbitrary cross section.

Further, in order to set the volume resistivity value ρv1 to be 60×10¹²Ωm or more and to set the ratio (ρv2/ρv1) to be 0.20 or more asdescribed above, it is preferable that the porosity is 1% or less(including 0) and the maximum diameter of the pores is 10 μm or less(including 0) when an arbitrary surface or cross section of the siliconnitride substrate is observed with an enlarged photograph (magnified by2,000 times or more).

The aforementioned enlarged photograph is an SEM photograph. In an SEMphotograph, pores are distinguishable because a different contrastdifference arises compared to the silicon nitride crystal grains and thegrain boundary phase. By making the proportion and size of poresobserved in an SEM photograph that is enlarged by magnifying by 2,000times or, further, 5,000 times, small, an excellent volume resistivityvalue can be obtained even under a high temperature environment (under a250° C. atmosphere).

Further, in a case where pores are present when an arbitrary crosssection is observed with an enlarged photograph, preferably the grainboundary phase is present at 10% or more of the circumferential lengthof the pores. Air is present in portions in which there are pores. Thesilicon nitride grains are insulators. Further, a grain boundary phasecomponent is formed by reaction of a sintering aid constituted by ametallic oxide. Therefore, because the grain boundary phase component isan oxide, the grain boundary phase component has insulating propertiesof a high level.

On the other hand, air is liable to become a passage for electricity. Inparticular, air is liable to become a passage for electricity when alarge voltage of 600 V or more is applied to the substrate. Pores areresidual defects from a densification process performed by a sinteringstep, and the densification progresses through the grain boundary phase.

Further, β-silicon nitride crystal grains have an elongated shape. Thestrength of the silicon nitride substrate is improved by randomlyorienting the β-silicon nitride crystal grains in an intricatelyintertwined state. On the other hand, when the β-silicon nitride crystalgrains are randomly oriented, gaps are liable to be formed at portionsbetween the silicon nitride crystal grains. It becomes difficult forpores to be formed if the gaps formed between silicon nitride crystalgrains are filled in with a grain boundary phase component. Furthermore,even if pores are formed, it is difficult for structural defects to beincluded that are caused by densification inhibition at the peripherythereof. Therefore, it is preferable that the circumference of the poresis covered with a grain boundary phase component since it suggests afavorable densification process.

Consequently, preferably, after setting the maximum diameter of pores to20 μm or less, a grain boundary phase component is caused to be presentat 10% or more of the outer circumferential length of the pores. Thelarger that the proportion of the outer circumferential length of thepore at which a grain boundary phase component is present, the betterthat the structure is, and the proportion is preferably equal to orgreater than 50% and less than or equal to 100%.

When the proportion of the outer circumferential length of the pores isincreased to be a large proportion of 50% or more, the dielectricstrength can be improved, and variations in the dielectric strength canbe decreased. In other words, even if pores exist, the dielectricstrength can be improved by covering the outer circumference of thepores with a grain boundary phase component.

Further, when a relative dielectric constant at 50 Hz is taken asε_(r50) and a relative dielectric constant at 1 kHz is taken asε_(r1000), it is preferable to satisfy a relation:(ε_(r50)-ε_(r1000))/εr_(r50)≦0.1. The term “relative dielectricconstant” refers to a value obtained when the electric capacity of acapacitor when a medium is filled between electrodes is divided by theelectric capacity thereof when there is a void (vacuum) between theelectrodes. The medium in this case is the silicon nitride substrate.When the relation: (ε_(r50)-ε_(r1000))/ε_(r50)≦0.1 is established, itindicates that the relative dielectric constant of the silicon nitridesubstrate does not become large even if the frequency becomes high. Thisindicates that the structure is one in which it is difficult forpolarization of the silicon nitride substrate to occur. Examples of asituation in which it is difficult for polarization to occur include onein which pores are small, or there are few pores. Furthermore, asdescribed above, it is also effective to control the size of the grainboundary phase, and to cause a grain boundary phase component to bepresent at the circumference of pores. In addition, it is also effectiveto reduce a segregated region that is described later on.

Further, when an arbitrary cross section of the silicon nitridesubstrate is observed, it is preferable that a maximum length of thesegregated region in the grain boundary phase is 5 μm or less (including0). The grain boundary phase is a reaction phase that adopts a sinteringaid as a main component. As described above, one or more kinds among thegroup consisting of rare earth elements, magnesium, titanium and hafniumare preferably selected as a sintering aid.

In this case, the term “segregated region” refers to a region in which adeviation of 30% or more arises with respect to the mean concentrationof a specific element when a unit area of 20 μm×20 μm is color mapped byEPMA (electron probe microanalysis). The term “specific element” refersto a sintering aid component. For example, in a case where yttrium oxide(Y₂O₃) is used as a sintering aid component, mapping of the Y element isperformed and a region in which the concentration deviates by 30% ormore with respect to the mean concentration is measured and determined.

In a case where a plurality of sintering aid components is used,metallic elements of the respective components are subjected to thecolor-mapping. For example, in a case of using the three kinds of oxidesY₂O₃, MgO and HfO₂ as sintering aid components, a region in which theconcentration deviates by 30% or more with respect to the meanconcentration is determined for “Y”, “MG” and “HF”. Note that, the term“deviate by 30% or more with respect to the mean concentration” refersto both a case in which the relevant amount is greater than the meanconcentration and a case in which the relevant amount is less than themean concentration.

It is preferable that the aforementioned segregated region is small. Themaximum length of the segregated region is preferably controlled to be 5μm or less, and more preferably to 1 μm or less (including 0). Bycontrolling the segregated region to be small, the volume resistivityvalue ρv1 can be set to 90×10¹² Ωm or more, and the aforementioned ratio(ρv2/ρv1) can be set to 0.40 or more.

Further, by realizing a state in which the maximum length of thesegregated region is 5 μm or less, and more preferably is 1 μm or less(including 0), variations in the dielectric strength can be also reducedto be 5% or less. As the thickness of a substrate is reduced to be thin,the influence of the thickness will be greater. Therefore, in the caseof a substrate whose thickness T1 is from 0.1 to 0.4 mm, it ispreferable to form a state in which the length of a segregated region is1 μm or less or a state in which a segregated region does not exist.

By adopting the above described configuration, even if the substratethickness T1 of the silicon nitride substrate is thinned to be athickness from 0.1 to 1.0 mm, or furthermore from 0.1 to 0.4 mm,variations in the dielectric strength can be reduced and the mean valueof the dielectric strength can be increased.

Further, by controlling the maximum length of the grain boundary phaseand the size of the silicon nitride crystal grains, not only thedielectric strength can be improved, but also the strength of thesubstrate can be made to be 600 MPa or more, after making the thermalconductivity of the silicon nitride substrate to be 50 W/m·K or more.

Further, by controlling the porosity, the pore size and the segregatedregion size (size of a segregated portion of a sintering aid), a furtherimprovement in the dielectric strength and improvement in the volumeresistivity value can be achieved.

In addition, by controlling a crystallized compound phase in a grainboundary phase to be 20% or more in terms of the area ratio as describedin Patent Document 2, it becomes easy to set the thermal conductivity to80 W/m·K or more, and furthermore, 90 W/m·K or more.

The silicon nitride substrate according to the embodiment is suitablefor a silicon nitride circuit board. A circuit board is a component onwhich a metallic plate and a metal layer are provided as circuitportions. Examples of the metallic plate having good electricalconductivity may include: a copper plate; and an Al plate. Further,various kinds of bonding methods such as the active metal brazing methodor direct bonding method can be applied for bonding of the metallicplate. Further, a metallic plate is also provided on the rear surface ofthe substrate as necessary. As examples of the metal layer, a metallizedlayer that is formed by heating a metallic paste, or a metallic thinfilm that is formed using a thin film formation technique such as anelectroplating method, a sputtering method or a thermal spraying may beadopted.

Furthermore, the silicon nitride substrate of this embodiment may alsobe used as a substrate for a pressure-contact structure as described inPatent Document 1. In particular, the silicon nitride substrateaccording to the embodiment is also effective as a substrate for apressure-contact structure, since the silicon nitride substrate isgreatly improved in the dielectric strength thereof.

Next, a method for manufacturing the silicon nitride substrate accordingto the embodiment will be described hereunder. Although a method formanufacturing the silicon nitride substrate is not particularly limitedas far as the silicon nitride substrate according to the embodiment hasthe above described structure, the following method is described as amethod for efficiently manufacturing the silicon nitride substrate.

First, a silicon nitride powder and a sintering aid powder are preparedas raw material powder. It is preferable that, with respect to thesilicon nitride powder, an a transformation rate is 80 mass % or more,an average grain diameter is 0.4 to 2.5 μm and an impurity oxygencontent is 2 mass % or less. It is also preferable that the impurityoxygen content is 2 mass % or less, more preferably 1.0 mass % or less,and further preferably is 0.1 to 0.8 mass %. If the impurity oxygencontent is a large content so as to exceed 2 mass %, there is a riskthat a reaction will occur between the impurity oxygen and the sinteringaid, and a grain boundary phase will be formed to a degree that is morethan necessary.

Further, the sintering aid is preferably a metallic oxide powder havingan average grain diameter of 0.5 to 3.0 μm. Oxides of rare earthelements such as magnesium, titanium and hafnium can be adopted asexample of the metallic oxide powder. Adding a sintering aid as ametallic oxide facilitates the formation of a liquid phase componentduring the sintering step.

Further, as the sintering aid, one or more kinds of elements selectedfrom the group consisting of rare earth elements, magnesium, titaniumand hafnium is added in a total amount of 2 to 14 mass % in terms ofoxide content. If the addition amount deviates from the aforementionedrange, the grain growth of the silicon nitride crystal grains as well asthe proportion of the grain boundary phase will deviate during thesintering step, and it will thus be difficult to make the ratio (T2/T1)fall within the target range.

Next, predetermined amounts of the silicon nitride powder and thesintering aid powder are mixed, and an organic binder is added to themixture to prepare a raw material mixture. At this time, as necessary,amorphous carbon, a plasticizer or the like may also be added. Theamorphous carbon functions as a deoxidizing agent. That is, theamorphous carbon reacts with oxygen and is discharged as CO₂ or CO tooutside, so that promotion of a liquid phase reaction of the sinteringstep is facilitated.

Next, a molding step of molding the raw material mixture is performed.As a method for molding the raw material mixture, a general-purposemolding-die pressing method, a cold isostatic pressing (CIP) method or asheet molding method such as the doctor-blade method or roll moldingmethod can be applied. As necessary, a solvent such as toluene, ethanolor butanol may be blended with the raw material mixture.

After the above described molding step, a degreasing step is performedon thus formed compact. In the degreasing step, the compact is heated toa temperature of 500 to 800° C. for one to four hours in a non-oxidizingatmosphere so as to degrease a large portion of the organic binder addedpreviously. A nitrogen gas atmosphere or an argon gas atmosphere may beadopted as examples of the non-oxidizing atmosphere.

Butyl methacrylate, polyvinyl butyral and polymethyl methacrylate may beadopted as example of the organic binder. When the total amount of thesilicon nitride powder and the sintering aid powder is taken as 100parts by mass, preferably the added amount of the organic binder is 3 to17 parts by mass.

If the addition amount of the organic binder is less than 3 parts bymass, the binder amount will be too small and it will be difficult tomaintain the shape of the compacts. In such a case, it will be difficultto improve mass productivity by stacking the compacts in multiple layers(tiers). On the other hand, if the binder amount is a large amount so asto exceed 17 parts by mass, there will be formed large cavities in therespective compacts after the degreasing step (compacts after thedegreasing treatment), so that the pores of the silicon nitridesubstrate will be large.

Next, the compact that was subjected to the degreasing treatment isaccommodated in a firing container and subjected to a heat treatmentstep in which the degreased compact is heated to a temperature of 1400to 1650° C. in a non-oxidizing atmosphere inside a calcining (sintering)furnace, and retained in the heated state for one to eight hours. Aliquid phase reaction of the sintering aid powder is promoted by thistreatment. By promoting the liquid phase reaction, diffusion of a liquidphase component to the grain boundary of the silicon nitride crystalgrains is promoted and pores are decreased.

If the retention temperature is less than 1400° C., it will be difficultfor a liquid phase reaction to occur, while if the retention temperatureexceeds 1650° C., because grain growth of the silicon nitride crystalgrains proceeds, an effect of decreasing pores by diffusion of a liquidphase component is not adequately obtained. Nitrogen gas (N₂) or argongas (Ar) may be adopted as examples of the non-oxidizing atmosphere. Itis also effective to stack the compacts in multiple layers to improvemass productivity. Further, by stacking the respective compacts inmultiple layers, the temperature inside the sintering furnace becomesuniform, so that the liquid phase reaction can thus be made uniform.

Next, a sintering step is performed. The sintering step is performed byheating the compact at a temperature of 1800 to 1950° C. in anon-oxidizing atmosphere for 8 to 18 hours. A nitrogen gas atmosphere ora reducing atmosphere including nitrogen gas is preferable as thenon-oxidizing atmosphere. Further, preferably the pressure inside thesintering furnace is a pressurized atmosphere.

If the compact is fired in a low temperature state in which thesintering temperature is less than 1800° C., the grain growth of thesilicon nitride crystal grains will be insufficient and it will bedifficult to obtain a dense sintered body. On the other hand, if thecompact is fired at a sintering temperature that is higher than 1950°C., there is a risk that the compact will decompose into Si and N₂ in acase where the atmospheric pressure inside the furnace is low, so thatit is preferable to control the sintering temperature to within theaforementioned range.

In a case where compacts are arranged in multiple layers (tiers) asdescribed above, the sintering temperature is preferably set to lessthan or equal to 1950° C. since there is a risk that pressure variationswill arise inside the sintering furnace. Further, if the sinteringtemperature is higher than 1950° C., there is a risk that the siliconnitride crystal grains will grow more than required, and the targetratio of (T2/T1) will not be obtained.

Further, it is preferable that a cooling rate of the sintered body afterthe sintering step is set to 100° C./h or less. By slowly (moderately)cooling the sintered body at a cooling rate of 100° C./h or less, orfurthermore, 50° C./h or less, the grain boundary phase can becrystallized. The proportion of a crystallized compound in the grainboundary phase can be increased. A liquid phase reaction of the grainboundary phase is promoted by the aforementioned heat treatment step.

Therefore, when crystallization of the grain boundary phase isperformed, the amount of aggregation and segregation of a liquid phasethat is generated in the sintered body is small, so that a grainboundary phase is obtained in which refined crystalline structures areuniformly dispersed. Further, pores that are formed in the crystallinestructures can also be made minute and simultaneously reduced.

Furthermore, by setting the cooling rate after the sintering step to100° C./h or less, the proportion of a crystallized compound phase inthe grain boundary phase can be made 20% or more in terms of the arearatio, and furthermore, can be made 50% or more. Due to crystallizationof the grain boundary phase, the thermal conductivity of the siliconnitride substrate can be increased to be 80 W/m·K or more.

Note that, if furnace cooling (natural cooling when the switch of thefurnace is turned off) is adopted with respect to the cooling rate afterthe sintering step, normally the cooling rate will be around 600° C./h.In such a case also, if the aforementioned heat treatment step isperformed, because uniformity of the grain boundary phase is achieved,upon making the thermal conductivity 50 W/m·K or more, theaforementioned ratio (T2/T1) and variations in the dielectric strengthcan be kept within the predetermined ranges.

Further, after the sintering step, it is also effective to perform anadditional heat treatment. It is desirable to perform the additionalheat treatment at a temperature that is equal to or higher than a liquidphase generating temperature and is lower than the treatment temperaturein the sintering step. It is also desirable to perform the additionalheat treatment under a pressurization condition. In the sintering step,a liquid phase component that is cooled from a surface activity state,namely, grain growth, enters a stationary state in the grain boundaryand is immobilized. However, stabilization from an active region isliable to proceed in a nonuniform manner. Hence, by performing heattreatment until a state in which a liquid phase is once more generatedand flows, while on the other hand until a state in which grain growthdoes not proceed, it becomes possible to more homogeneously improvestabilization of a grain boundary by the cooling thereafter.

Further, when performing an additional heat treatment, it is effectiveto press the sintered body and to flip over the rear and front surfacesof the sintered body and the like. By performing the additional heattreatment, pores in the silicon nitride substrate can be eliminated, thepores can be made smaller, and a state can be entered in which a grainboundary phase component is present along the circumferential length ofpores. Preferably, the temperature for the aforementioned heat treatmentis from 1000° C. or more to 1700° C. or less.

By performing the heat treatment at a temperature in the aforementionedrange from 1000° C. to 1700° C., the grain growth of the silicon nitridecrystal grains can be suppressed, and a grain boundary phase componentcan be moved somewhat. At such time, by performing pressing and flippingover the rear and front surfaces, obtainment of effects such that poresare eliminated, pores are reduced, and a state is entered in which agrain boundary phase component is present along the circumferentiallength of pores is easily obtained.

If the manufacturing method described in the foregoing is used, thesilicon nitride substrate according to the embodiment can be easilyobtained.

EXAMPLES Examples 1 to 20 and Comparative Example 1

As a silicon nitride powder, a powder was prepared in which the averagegrain diameter was 1.0 μm, an impurity oxygen content was 1 mass %, andan a transformation rate was 98%. Further, the substances shown in Table1 and Table 2 were prepared as sintering aid powder. Note that, thesintering aid powder that was prepared had an average grain diameter of0.8 to 1.6 μm.

The silicon nitride powder and sintering aid powder were mixed toprepare a raw material mixture. A dispersing agent and an organicsolvent were blended into the raw material mixture, and ball-mill mixingwas performed. Next, 10 parts by mass of butyl methacrylate as anorganic binder and four parts by mass of dibutyl phthalate as aplasticizer were added to 100 parts by mass of the raw material powdermixture, the mixture was mixed, an organic solvent was additionallyadded, and then ball-mill mixing was sufficiently performed to prepare aslurry-like raw material mixture. The slurry viscosity was adjusted to5000 to 15000 CPS, and thereafter the raw material mixture was moldedinto sheets by a sheet molding method (doctor-blade method) and dried tothereby prepare compacts (green sheets).

The compacts were heated for 1 to 4 hours at a temperature of 500 to800° C. in a nitrogen gas atmosphere and then subjected to a degreasingstep.

Next, the heat treatment steps and sintering steps shown in Table 1 andTable 2 were performed on the compacts that had undergone the degreasingtreatment. After performing these steps, silicon nitride substrates ofthe examples and comparative example were prepared under the conditionsshown in Table 1 and Table 2. Further, the heat treatment step andsintering step were executed in a state in which the compacts werestacked in multiple layers (stacks of 10 tiers).

TABLE 1 Composition Silicon Sintering Substrate Size Sintering ConditionNitride Aid Length × Width × Upper Column: Heat Treatment Process SampleNo. (mass %) (mass %) Thickness (mm) Lower Column: Sintering ProcessExample 1 (95) Y₂O₃ (3) 50 × 50 × 0.635 1500° C. × 2 hr in Nitrogen GasAtmosphere MgO (2) 1900° C. × 9 hr in Nitrogen Gas Atmosphere →FurnaceCooling (600° C./hr) Example 2 (91) Y₂O₃ (3) 50 × 50 × 0.635 1550° C. ×5 hr in Nitrogen Gas Atmosphere Er₂O₃ (4) 1850° C. × 10 hr in NitrogenGas Atmosphere HfO₂ (1) →Cooling Rate 50° C./hr MgO (1) Example 3 (89)Y₂O₃ (7) 50 × 30 × 0.32 1430° C. × 8 hr in Nitrogen Gas Atmosphere HfO₂(2) 1800° C. × 12 hr in Nitrogen Gas Atmosphere MgO (1) →Cooling Rate100° C./hr TiO₂ (1) Example 4 (93) Y₂O₃ (3) 50 × 40 × 0.32 1600° C. × 6hr in Nitrogen Gas Atmosphere MgO (4) 1950° C. × 8 hr in Nitrogen GasAtmosphere →Furnace Cooling (600° C./hr) Example 5 (87) Y₂O₃ (4) 60 × 40× 0.20 1650° C. × 2 hr in Nitrogen Gas Atmosphere Er₂O₃ (5) 1870° C. ×15 hr in Nitrogen Gas Atmosphere HfO₂ (2) →Cooling Rate 20° C./hr TiO₂(1) MgO (1) Example 6 (93) Y₂O₃ (3) 60 × 40 × 0.20 1500° C. × 5 hr inNitrogen Gas Atmosphere MgO (3) 1900° C. × 11 hr in Nitrogen GasAtmosphere HfO₂ (1) →Cooling Rate 100° C./hr

TABLE 2 Composition Silicon Sintering Substrate Size SinteringConditions Nitride Aid Length × Width × Upper Column: Heat TreatmentProcess Sample No. (mass %) (mass %) Thickness (mm) Lower Column:Sintering Process Example 7 (88) Y₂O₃ (3) 50 × 50 × 0.15 1560° C. × 5 hrin Nitrogen Gas Atmosphere Er₂O₃ (5) 1830° C. × 10 hr in Nitrogen GasAtmosphere HfO₂ (1) →Cooling Rate 40° C./hr TiO₂ (2) MgO (1) Example 8(87) Y₂O₃ (8) 50 × 30 × 0.25 1400° C. × 4 hr in Nitrogen Gas AtmosphereHfO₂ (2) 1880° C. × 13 hr in Nitrogen Gas Atmosphere TiO₂ (2) →CoolingRate 80° C./hr MgO (1) Example 9 (88.9) Y₂O₃ (4) 40 × 40 × 0.20 1520° C.× 2 hr in Nitrogen Gas Atmosphere Er₂O₃ (4) 1870° C. × 18 hr in NitrogenGas Atmosphere HfO₂ (2) →Cooling Rate 80° C./hr TiO₂ (0.1) MgO (1)Example 10 (87.5) Y₂O₃ (4) 70 × 50 × 0.25 1610° C. × 7 hr in NitrogenGas Atmosphere Er₂O₃ (5) 1930° C. × 16 hr in Nitrogen Gas AtmosphereHfO₂ (2) →Cooling Rate 20° C./hr TiO₂ (0.5) MgO (1) Comparative (92)Y₂O₃ (3) 50 × 50 × 0.635 Heat Treatment Process (None) Example 1 MgO (5)1850° C. × 10 hr in Nitrogen Gas Atmosphere →Cooling Rate 100° C./hr

With respect to the silicon nitride substrates according to each exampleand the comparative example, the thermal conductivity, three-pointbending strength and sectional structure in the substrate thicknessdirection were observed, and the ratio (T2/T1), maximum diameter of thegrain boundary phase in the thickness direction, average grain diameterwith respect to the long diameter of the silicon nitride crystal grains,and the porosity were examined. The pore size and a segregated region inthe grain boundary phase were also examined.

Note that, the aforementioned thermal conductivity was determined by alaser flash method. The three-point bending strength was measured inaccordance with JIS-R-1601 (2008). The substrate thickness T1 wasmeasured with a calipers. The porosity was determined by the mercurypenetration method. Further, an SEM photograph (magnification of 2,000times) of an arbitrary sectional structure was photographed in thesubstrate thickness direction, and the maximum diameter of a grainboundary phase in the thickness direction as well as the average graindiameter with respect to the long diameter of the silicon nitridecrystal grains were examined.

Further, an enlarged photograph (SEM photograph with a magnification of5000 times) of a unit area of 20 μm×20 μm at an arbitrary cross sectionin the substrate thickness direction was photographed at 10 locations,and pore sizes (maximum diameter) were determined. Further, unit areasof 20 μm×20 μm were subjected to color mapping with regard to metallicelements of sintering aid components by EPMA. The unit areas of 20 μm×20μm were measured at five locations, and the mean concentration and alsothe size of segregated regions (regions at which the concentration ofthe metallic element deviated by 30% or more) were determined. Theresults are shown in Table 3 and Table 4 below.

TABLE 3 SEM Photograph (2000 Magnifications) Maximum Three Length ofGrain Average Long Point Boundary Phase Grain Diameter Thermal Bendingin Thickness of Silicon Nitride Conductivity Strength Porocity RatioDirection Crystal Grains Sample No. (W/m · K) (MPa) (%) (T2/T1) (μm)(μm) Example 1 90 600 1.0 0.08 14 10 Example 2 85 700 0.3 0.25 10 5Example 3 80 650 0.0 0.20 4 3 Example 4 85 660 0.5 0.10 7 7 Example 5 95680 0.0 0.17 3 4 Example 6 90 720 0.3 0.13 5 5 Example 7 98 750 0.0 0.193 3 Example 8 92 700 0.2 0.17 6 6 Example 9 90 730 0.2 0.20 4 5 Example10 95 700 0.0 0.22 4 5 Comparative 80 600 1.0 0.42 54 14 Example 1

TABLE 4 SEM Photograph EPMA Color Mapping (5000 Magnifications) MaximumLength of Pore Size Segregated Region in Grain Sample No. (MaximumDiameter) (μm) Boundary Phase (μm) Example 1 3.0 2.0 Example 2 1.0 1.5Example 3 0.2 None Example 4 0.5 1.0 Example 5 0.2 None Example 6 0.20.5 Example 7 0.2 None Example 8 0.4 0.8 Example 9 0.2 0.6 Example 100.2 None Comparative 24 4  Example 1

Examples 11 to 20

Next, the additional heat treatments shown in Table 5 were performed onthe silicon nitride substrates of Examples 1 to 10.

TABLE 5 Silicon Nitride Sample No. Substrate Additional Heat TreatmentExample 11 Example 1 Pressing, 1600° C. × 1 hr Example 12 Example 2Pressing, 1000° C. × 2 hr Example 13 Example 3 Flipping over, 1100° C. ×5 hr Example 14 Example 4 Flipping over, 1000° C. × 3 hr Example 15Example 5 Pressing, 1050° C. × 3 hr Example 16 Example 6 Pressing, 1300°C. × 2 hr Example 17 Example 7 Flipping over, 1400° C. × 3 hr Example 18Example 8 Flipping over, 1350° C. × 2 hr Example 19 Example 9 Pressing,1000° C. × 5 hr Example 20 Example 10 Pressing, 1100° C. × 2 hr

The pore size (maximum diameter) and the proportion at which a grainboundary phase component was present at the circumferential length of apore were determined for the silicon nitride substrates of Examples 1 to20 and Comparative Example 1. The pore size (maximum diameter) wasdetermined using an SEM photograph (magnification of 5,000 times). Theproportion at which a grain boundary phase component was present at thecircumferential length of a pore was determined by EPMA. The results areshown in Table 6 below.

TABLE 6 Proportion at which Grain Pore Size Boundary Phase Component(Maximum Diameter) is present at Circumferential Sample No. (μm) Lengthof Pore (%) Example 1 3.0 47 Example 2 1.0 77 Example 3 0.2 73 Example 40.5 55 Example 5 0.2 79 Example 6 0.2 63 Example 7 0.2 82 Example 8 0.485 Example 9 0.2 87 Example 10 0.2 84 Example 11 1.9 66 Example 12 0.298 Example 13 None — Example 14 0.1 75 Example 15 None — Example 16 None— Example 17 None — Example 18 0.1 100  Example 19 None — Example 20None —

As is apparent from the results shown in the above Table 6, in thesilicon nitride substrates according to the respective examples, thepores were small and a grain boundary phase component was present at 10%or more of the circumferential length of the pores. Further, byperforming the additional heat treatment, the pores could be madesmaller (including cases where no pores existed).

The dielectric strength and volume resistivity value were measured forthe silicon nitride substrates according to the above described examplesand comparative example. The aforementioned dielectric strength wasmeasured by the four-terminal method in accordance with JIS-C-2141.Spherical electrodes that each had a tip with a diameter of 20 mm wereused as the measuring terminals. Further, the measurement of thedielectric strength was performed in Fluorinert. The mean value andvariation were determined at the five measurement locations shown inFIG. 4 (S1 to S5).

The volume resistivity value was measured in accordance with JIS-K-6911.A front surface-side carbon electrode was disk-shaped with a diameter of20 mm, and a rear surface-side carbon electrode was disk-shaped with adiameter of 28 mm. The applied voltage was 1000 V. A volume resistivityvalue ρv1 at room temperature (25° C.), and a volume resistivity valueρv2 at 250° C. were measured.

The frequency dependency of the relative dielectric constant was alsoinvestigated. The frequency dependency of the relative dielectricconstant was determined by (ε_(r50)-ε_(r1000))/ε_(r50) when taking therelative dielectric constant at 50 Hz as ε_(r50), and taking therelative dielectric constant at 1 kHz as ε_(r1000). The results areshown in Table 7 and Table 8.

TABLE 7 Frequency Dielectric Volume Resistivity Dependency of StrengthValue (×10¹² Ωm) Relative Mean ρv1 Room Dielectric Value Variation Temp.ρv2 Constant Sample No. (kV/mm) (%) (25° C.) (250° C.) ρv2/ρv1 (ε_(r50)− ε_(r1000))/ε_(r50) Example 1 17 14 100 22 0.22 0.10 Example 2 22 10150 45 0.30 0.07 Example 3 23 5 200 90 0.45 0.04 Example 4 20 8 170 540.32 0.05 Example 5 24 5 210 107 0.50 0.03 Example 6 20 7 180 63 0.350.02 Example 7 23 4 220 117 0.53 0.01 Example 8 20 6 200 80 0.40 0.05Example 9 21 6 190 80 0.42 2.04 Example 10 24 4 190 93 0.49 0.03Comparative 15 22 55 9 0.17 0.24 Example 1

TABLE 8 Frequency Dependency of Dielectric Volume Resistivity RelativeStrength Value (×10¹² Ωm) Dielectric Mean ρv1 Room Constant ValueVariation Temp. ρv2 (ε_(r50) − ε_(r1000))/ Sample No. (kV/mm) (%) (25°C.) 250° C. ρv2/ρv1 ε_(r50) Example 11 20 10 130 40 0.31 0.08 Example 1225 7 185 93 0.50 0.03 Example 13 26 2 240 156 0.65 0.02 Example 14 27 5180 77 0.43 0.02 Example 15 28 2 250 173 0.69 0.01 Example 16 25 4 200104 0.52 0.008 Example 17 27 2 260 182 0.70 0.005 Example 18 28 3 255153 0.60 0.02 Example 19 27 3 230 129 0.56 0.01 Example 20 29 2 250 1580.63 0.01

As described above, the silicon nitride substrates according to therespective examples exhibited excellent characteristics with respect todielectric strength and volume resistivity values. The silicon nitridesubstrates also exhibited excellent characteristics in relation tofrequency dependency of the relative dielectric constant.

Such a silicon nitride substrate has excellent insulating propertieseven if the silicon nitride substrate is thinned. Therefore, the siliconnitride substrate can ensure excellent reliability even when applied toa silicon nitride circuit board or a substrate for a pressure-contactstructure.

Although several embodiments of the present invention have beenillustrated above, these embodiments have been presented by way ofexample only, and are not intended to limit the scope of the invention.Indeed, these novel embodiments may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the embodiments described herein may be made without departingfrom the gist of the invention. These embodiments and the modificationsthereof are included within the scope and gist of the invention, and arealso included in the scope of the inventions described in theaccompanying claims and their equivalents. Further, the respectiveembodiments described above may be embodied by combining the embodimentswith each other.

REFERENCE SIGNS LIST

-   1 . . . Silicon nitride substrate-   2 . . . Silicon nitride crystal grain-   3 . . . Grain boundary phase-   4, 5 . . . Measuring terminals used in four-terminal method-   6 . . . Dielectric strength measurement instrument-   7, 8 . . . Carbon electrode-   9 . . . Volume resistivity value measurement instrument

The invention claimed is:
 1. A silicon nitride substrate comprisingsilicon nitride crystal grains and a grain boundary phase and having athermal conductivity of 50W/m·K or more, wherein, in a sectionalstructure of the silicon nitride substrate, a ratio, T2/T1, of a totallength T2 of the grain boundary phase in a thickness direction withrespect to a thickness T1 of the silicon nitride substrate is 0.01 to0.30, an average grain diameter with respect to a long diameter of thesilicon nitride crystal grains is between 1.5 and 10 μm, and a variationfrom a dielectric strength mean value when measured by a four-terminalmethod in which electrodes are brought into contact with front and rearsurfaces of the substrate is 20% or less.
 2. The silicon nitridesubstrate according to claim 1, wherein a variation in the dielectricstrength is 15% or less.
 3. The silicon nitride substrate according toclaim 1, wherein the dielectric strength mean value is 15 kV/mm or more.4. The silicon nitride substrate according to claim 1, wherein a volumeresistivity value when a voltage of 1000 V is applied at 25° C. is 60×10¹² Ωm or more.
 5. The silicon nitride substrate according to claim 1,wherein a ratio, ρv2/ρv1, between a volume resistivity value pv1 when avoltage of 1000 V is applied at 25° C. and a volume resistivity valueρv2 when a voltage of 1000 V is applied at 250° C. is 0.20 or more. 6.The silicon nitride substrate according to claim 1, wherein, when arelative dielectric constant at 50Hz is represented by ε_(r50) and arelative dielectric constant at 1 kHz is represented by ε_(r1000),(ε_(r50)-ε_(r1000))/ε_(r50)≦0.1.
 7. The silicon nitride substrateaccording to claim 1, wherein, when a cross section in a thicknessdirection of the silicon nitride substrate is observed with an enlargedphotograph, a maximum length of the grain boundary phase is 50 μm orless.
 8. The silicon nitride substrate according to claim 1, wherein aporosity of the silicon nitride substrate is 3% or less.
 9. The siliconnitride substrate according to claim 1, wherein, when an arbitrarysurface or cross section of the silicon nitride substrate is observedwith an enlarged photograph, a maximum diameter of a pore is 0 μm ormore and no more than 20 μm.
 10. The silicon nitride substrate accordingto claim 1, wherein the substrate has pores, and when an arbitrary crosssection of the silicon nitride substrate is observed with an enlargedphotograph, a maximum diameter of a pore is greater than 0 μm and nomore than 20 μm, and a grain boundary phase component is present at 10%or more of a circumferential length of a pore.
 11. The silicon nitridesubstrate according to claim 1, wherein, when an arbitrary cross sectionof the silicon nitride substrate is observed, a maximum length of asegregated region in the grain boundary phase is 0 μm or more and nomore than 5 μm.
 12. The silicon nitride substrate according to claim 1,wherein the thickness T1 of the silicon nitride substrate is from 0.1 to1.0 mm.
 13. The silicon nitride substrate according to claim 1, wherein,in terms of an area ratio, 20% or more of the grain boundary phase is acrystallized compound phase.
 14. A silicon nitride circuit board inwhich a circuit portion is provided on a silicon nitride substrateaccording to claim
 1. 15. A silicon nitride substrate comprising siliconnitride crystal grains and a grain boundary phase and having a thermalconductivity of 50 W/m·K or more, wherein, in a sectional structure ofthe silicon nitride substrate, a ratio, T2/T1, of a total length T2 ofthe grain boundary phase in a thickness direction with respect to athickness T1 of the silicon nitride substrate is 0.01 to 0.30, avariation from a dielectric strength mean value when measured by afour-terminal method in which electrodes are brought into contact withfront and rear surfaces of the substrate is 20% or less, and, in termsof an area ratio, 20% or more of the grain boundary phase is acrystallized compound phase.